Isocyanates are widely used as raw materials of such products as polyurethane foam, paints, adhesives and the like. The main industrial production process of isocyanates involves reacting amine compounds with phosgene (phosgene method), and nearly the entire amount of isocyanates produced throughout the world are produced according to the phosgene method. However, the phosgene method has numerous problems.
Firstly, this method requires the use of a large amount of phosgene as the raw material. Phosgene is extremely toxic and requires special handling precautions to prevent exposure of handlers thereof, and also requires special apparatuses to detoxify waste.
Secondly, since highly corrosive hydrogen chloride is produced in large amounts as a by-product of the phosgene method, in addition to requiring a process for detoxifying the hydrogen chloride, in many cases hydrolytic chlorine is contained in the isocyanates produced, which may have a detrimental effect on the weather resistance and heat resistance of polyurethane products in the case of using isocyanates produced using the phosgene method.
On the basis of this background, a process for producing isocyanate compounds has been sought that does not use phosgene. One example of a method for producing isocyanate compounds without using phosgene that has been proposed involves thermal decomposition of carbamic acid esters. Isocyanates and hydroxy compounds have long been known to be obtained by thermal decomposition of carbamic acid esters (see, for example, Non-Patent document 1). The basic reaction is illustrated by the following formula:R(NHCOOR′)a→R(NCO)a+a R′OH  (1)(wherein R represents an organic residue having a valence of a, R′ represents a monovalent organic residue, and a represents an integer of from 1 or more).
Among the carbamic acid esters, aryl carbamates, which are carbamic acid esters in which the ester group is an aromatic group, offer the advantage of allowing the temperature of the thermal decomposition reaction to be set to a lower temperature than alkyl carbamates in which the ester group is an alkyl group (see Patent document 1).
Various processes have been disclosed thus far for producing aryl carbamates.
According to the description of Patent document 2, it is described that corresponding aryl alkyl monocarbamates are obtained at a yield of from 90 to 95% by reacting alkyl monoamines with diaryl carbonates in the presence of a solvent such as benzene, dioxane or carbon tetrachloride. In addition, a process has been proposed in Patent document 3 for continuously producing methyl carbamic acid phenyl ester from methyl amine and diphenyl carbonate.
However, all of these processes are processes for producing alkyl aryl carbamates using lower alkyl monoamines as amines, and not aryl alkyl polycarbamates. In the case of producing the corresponding aryl alkyl polycarbamic acid esters from alkyl polyamines such as alkyl diamines or alkyl triamines, completely different problems arise from the case of using alkyl monoamines. This is because, although only urea compounds are produced as by-products by side reactions represented by the following formula (3) and/or formula (4) in addition to the reaction represented by the following formula (2) in the case of using the alkyl monoamines, in the case of the alkyl polyamines such as alkyl diamines or alkyl triamines, extremely numerous types of urea compounds are produced as by-products, such as compounds represented by the following formula (5), formula (6) and/or formula (7).
(wherein R′ represents a monovalent alkyl group or an aromatic group, Ar represents a monovalent aromatic group, and p, q and r respectively represent an integer of 1 or more).
Namely, reactions resulting in the production of by-products in the form of these various urea compounds cause the problem of decreasing yield of the target compound in the form of the aryl alkyl polycarbamates, as well as the problem of making it extremely difficult to separate and purify the target product from the mixture of these urea compounds and polyurea compounds.
On the basis thereof, although extremely few attempts have been made to produce aryl alkyl polycarbamic acid esters from alkyl polyamines and diaryl carbonates, a very small number of attempts have been reported. For example, according to the specification of Patent document 4, a process has been proposed for obtaining 1,6-hexamethylene dicarbamic acid phenyl ester in a reaction system in which a solution, in which 1 mole of 1,6-hexamethylene diamine is dissolved in 5-times moles of benzene, is dropped into a solution, in which 1 mole of diphenyl carbonate is dissolved in 5-times moles of benzene, while stirring at 80° C. According to this patent specification, it is important to use solvents in which the 1,6-hexamethylene dicarbamic acid phenyl ester dissolves as little as possible for the reaction solvent in order to allow the reaction to proceed advantageously, and solvents such as benzene or chlorobenzene are described as being preferable examples of such solvents.
From this viewpoint, the target 1,6-hexamethylene dicarbamic acid phenyl ester is obtained in Non-Patent document 3 by carrying out a reaction between 0.01 mole of diphenyl carbonate and 0.005 moles of 1,6-hexamethylene diamine using 40 mL of toluene for the reaction solvent for the long period of time of 20 hours. However, the yield is only 93% despite the use of this large amount of toluene, and the problem of the production of by-products in the form of urea compounds and polyurea compounds that must be separated remains.
In addition, Patent document 5 discloses a production process of diurethane compounds in which diaryl carbonates and amine compounds are reacted in the presence of protic acids. However, in the case of carrying out the production process disclosed in this patent publication industrially, the yield of the diurethane compound cannot be said to be adequate and it is necessary to carry out the reaction at a low temperature to inhibit side reactions, thereby resulting in the disadvantage of a long reaction time.
Patent document 6 describes a process in which diaryl carbonates and aromatic polyamines are reacted in the presence of heterocyclic tertiary amines such as 2-hydroxypyridine. In addition to this process requiring an expensive catalyst equal to or greater than an equimolar amount based on the reaction substrate, it also has the problem of the reaction rate being low.
According to Patent document 7, although a process is described for synthesizing aromatic urethanes at a temperature of from 140 to 230° C. in the presence of aromatic amines, diaryl carbonate and Lewis acid catalyst, in the case of this process as well, the use of a Lewis acid causes corrosion of the apparatus and separation and recovery of the product is difficult.
In Patent document 8, a production process of alkyl polycarbamic acid aryl esters is disclosed comprising carrying out reaction in a substantially homogeneous solution state using from 1 to 3 equivalents of diaryl carbonate per equivalent of alkyl polyamine amino groups and using aromatic hydroxy compounds for the reaction solvent when producing alkyl polycarbamic acid aryl esters by reacting alkyl polyamines and diaryl carbonates. According to this patent publication, alkyl polycarbamic acid aryl esters are obtained at high selectivity and a high yield of generally 96% or more, and 98% or more in a preferable aspect thereof. However, since the formation of urea compounds has been confirmed, albeit in small amounts, the formation of urea compounds cannot be completely avoided.
On the other hand, thermal decomposition of carbamic acid esters is susceptible to the simultaneous occurrence of various irreversible side reactions such as thermal denaturation reactions undesirable for carbamic acid esters or condensation of isocyanates formed by the thermal decomposition. Examples of these side reactions include a reaction in which urea bonds are formed as represented by the following formula (8), a reaction in which carbodiimides are formed as represented by the following formula (9), and a reaction in which isocyanurates are formed as represented by the following formula (10) (see Non-Patent document 1 and Non-Patent document 2).

In addition to these side reactions leading to a decrease in yield and selectivity of the target isocyanates, in the production of polyisocyanates in particular, these reactions may make long-term operation difficult as a result of, for example, causing the precipitation of polymeric solids that clog the reaction vessel.
Various methods have been proposed for producing isocyanates using carbamic acid esters as raw materials.
According to Patent document 9, aromatic diisocyanates and/or polyisocyanates are produced by going through the following two steps. More specifically, in the first step, aromatic primary amines and/or aromatic primary polyamines are reacted with O-alkyl carbamates in the presence or absence of a catalyst and in the presence or absence of urea and alcohol to form aryl diurethanes and/or aryl polyurethanes followed by removal of the ammonia formed as necessary. In the second step, aromatic isocyanates and/or aromatic polyisocyanates are obtained by thermal decomposition of the aryl diurethanes and/or aryl polyurethanes.
Several processes are known for forming the corresponding isocyanates and alcohols by thermal decomposition of (cyclic) aliphatic, and particularly aromatic monourethanes and diurethanes, examples of which include a process carried out at a high temperature in a gaseous phase, and a process carried out under comparatively low temperature conditions in a liquid phase. However, since there are cases in which, for example, the reaction mixture forms precipitates, polymeric substances and closed compounds in the reaction vessel and recovery apparatus due to the occurrence of side reactions as previously described, or these substances form substances that adhere to the walls of the reaction vessel, economic efficiency is poor in the case of producing isocyanates over a long period of time.
Thus, chemical methods, such as the use of a special catalyst (see Patent document 10 and Patent document 11) or a catalyst combined with an insert solvent (see Patent document 12) are disclosed for improving yield during thermal decomposition of urethane.
More specifically, Patent document 13 describes a process for producing hexamethylene diisocyanate involving thermal decomposition of hexamethylene diethyl urethane in the presence of dibenzyl toluene used as a solvent and in the presence of a catalyst mixture containing methyl toluene sulfonate and diphenyl tin dichloride. However, since there is no detailed description of production of the starting components, isolation or purification and arbitrary recovery of the solvent and catalyst mixture, the economic effects of this process were unable to be assessed.
According to the process described in Patent document 14, urethane can be easily decomposed to isocyanate and alcohol in a carbon-containing fluidized bed without using a catalyst. In addition, according to the description of Patent document 15, hexamethylene dialkyl urethane can be decomposed in a gaseous phase at a temperature exceeding 300° C. in the presence or absence of a gas-permeable packaging material composed of, for example, carbon, copper, bronze, steel, zinc, aluminum, titanium, chromium, cobalt or quartz, resulting in the formation of hexamethylene diisocyanate. According to the description of Patent document 14, the process is carried out in the presence of a hydrogen halide and/or hydrogen halide donor. However, this process is unable to achieve a yield of hexamethylene diisocyanate of 90% or more. This is because the decomposition product partially rebonds resulting in the formation of urethane bonds. Thus, purification of the hexamethylene diisocyanate by distillation is still required, and there are numerous cases in which yield loss increases.
Moreover, Patent document 16 discloses that monocarbamates can be advantageously decomposed at high yield without using a solvent under a reduced pressure and/or in the presence of absence of a stabilizer and at a comparatively low temperature. The decomposition products (monoisocyanates and alcohols) are removed by distillation from a boiling reaction mixture and captured separately by fractional condensation. A method for partially removing the reaction mixture is generically described in order to remove by-products formed during thermal decomposition. Thus, although it is possible to remove by-products from the bottom of the reaction vessel, the problem of the case of substances adhering to the walls of the reaction vessel as previously described remains, and problems regarding long-term operation are unresolved. In addition, there is no description regarding the industrial use of the removed residual substances (containing large amounts of useful components).
According to the description of Patent document 17, thermal decomposition of aliphatic, alicyclic or aromatic polycarbamates is carried out at from 150 to 350° C. and from 0.001 to 20 bar, in the presence of an inert solvent, and in the presence or absence of a catalyst and assistant in the form of hydrogen chloride, organic acid chloride, alkylation agent or organic tin compound. By-products formed can be removed continuously from the reaction vessel together with the reaction solution, for example, and corresponding amounts of fresh solvent or recovered solvent are added simultaneously. Examples of disadvantages of this process include a decrease in the space time yield of polyisocyanate due to the use of a circulating solvent, and a large energy requirement, including recovery of the solvent. Moreover, since the assistant used is volatile under the reaction conditions, contamination of the decomposition products can occur. In addition, since there is a large amount of residual substances formed relative to the polyisocyanate formed, there is some doubt regarding economic efficiency and reliability as an industrial process.
Patent document 18 describes a process for continuous thermal decomposition of carbamates supplied along the inner walls of a tubular reaction vessel in the form of a liquid in the presence of a high boiling point solvent, an examples of which may include 5-(ethoxycarbonylamino)-1-(ethoxycarbonylaminomethyl)-1,3,3-trimethylcyclohexane as the alicyclic diurethane. This process has the shortcomings of low yield and low selectivity during production of (cyclic) aliphatic diisocyanates. In addition, there is no description of a continuous method accompanying recovery of rebonded or partially decomposed carbamates, nor is there any mention of post-treatment of solvent containing by-products and catalyst.
The production of isocyanates using diaryl carbonates and amino compounds as raw materials can easily be imagined to be possible by combining the aryl carbamate production processes and isocyanate production processes using thermal decomposition of carbamic acid esters as described above. However, in order to combine these aryl carbamate production processes and isocyanate production processes using thermal decomposition of aryl carbamates as described above, methods involving a complex procedure consisting of carrying out the thermal decomposition of aryl carbamate by reacting diaryl carbonates and amine compounds and separating the aryl carbamates from the resulting reaction solution followed by thermal decomposition of the aryl carbamates, or methods using the reaction solution obtained during production of aryl carbamates directly in the thermal decomposition reaction, must be employed.
In this regards, Patent document 19 discloses a process for synthesizing aromatic isocyanates by synthesizing urethane compounds by reacting aromatic amines and diaryl carbonates in the presence of a Lewis acid catalyst and continuing with thermal decomposition of the urethane compounds in the diaryl carbonates used to synthesize the urethane compounds. In this patent publication, isocyanates are produced by applying a urethane-containing reaction solution obtained by reacting amine compounds and diaryl carbonates in the presence of a Lewis acid catalyst to a thermal decomposition reaction in the reaction vessel used for the urethane synthesis.    Patent document 1: U.S. Pat. No. 3,992,430    Patent document 2: Japanese Patent Application Laid-open No. S52-71443    Patent document 3: Japanese Patent Application Laid-open No. S61-183257    Patent document 4: German Patent No. 925496    Patent document 5: Japanese Patent Application Laid-open No. H10-316645    Patent document 6: Japanese Patent Application Laid-open No. S52-136147    Patent document 7: Japanese Patent Application Laid-open No. 2004-262834    Patent document 8: Japanese Patent Application Laid-open No. H1-230550    Patent document 9: U.S. Pat. No. 4,290,970    Patent document 10:U.S. Pat. No. 2,692,275    Patent document 11: U.S. Pat. No. 3,734,941    Patent document 12: U.S. Pat. No. 4,081,472    Patent document 13: U.S. Pat. No. 4,388,426    Patent document 14: U.S. Pat. No. 4,482,499    Patent document 15: U.S. Pat. No. 4,613,466    Patent document 16:U.S. Pat. No. 4,386,033    Patent document 17:U.S. Pat. No. 4,388,246    Patent document 18: U.S. Pat. No. 4,692,550    Patent document 19: Japanese Patent Application Laid-open No. 2004-262835    Non-Patent document 1: Berchte der Deutechen Chemischen Gesellschaft, Vol. 3, p. 653, 1870    Non-Patent document 2: Journal of American Chemical Society, Vol. 81, p. 2138, 1959    Non-Patent document 3: Journal of Polymer Science, Polymer Chemistry Edition, Vol. 17, p. 835, 1979